1. Field of the Invention
The invention relates to an apparatus for reducing a magnetic unidirectional flux component in the core of a transformer, i.e., a three-phase transformer, comprising a plurality of compensation windings that are magnetically coupled to the core of the transformer when used in transformers in the low voltage or medium voltage range and transformers of very high power (power transformers, high voltage direct current transmission transformers).
2. Description of the Related Art
In electrical transformers, as used in energy distribution networks, the situation may arise that direct current is fed into the primary winding or secondary winding which is undesirable. Such a direct current feed as (the DC-component) may, for example, originate from conventional electronic structural components presently in use when controlling electrical drives or even in reactive power compensation. A further cause may be geomagnetically induced currents (GIC).
Due to solar winds, the earth's magnetic field is altered and thus very low frequency voltages are induced on conductor loops on the earth's surface. With long electrical energy transmission lines the induced voltage may cause relatively large low frequency currents (quasi direct currents). Geomagnetically induced currents occur approximately in ten-year cycles. They are evenly distributed on all (three) phases, may reach up to 30 A per phase and may be discharged via the neutral point of a transformer. This leads to considerable saturation of the core of the transformer in a half-cycle and therefore to a high excitation current in a half-cycle. This additional excitation has a large harmonic component and as a result, via the stray field with the harmonic component, eddy current losses are produced in the windings and core components of the transformer. This may lead to local overheating in the transformer. Moreover, due to the high excitation requirement this leads to a high consumption of reactive power and a drop in voltage. Collectively, this may lead to instability of the energy transmission network. Put simply, the transformer behaves in a half-wave in the manner of a reactor.
Some energy transmission companies, therefore, already require in the specification of transformers 100 A direct current for the neutral point of the transformer.
As disclosed in WO 2012/041368 A1, use is made of an electrical voltage induced in a compensation winding and the electrical voltage is used for the compensation of the interfering magnetic unidirectional flux component by a thyristor switch being connected in series with a current limiting reactor, in order to introduce the compensation current into the compensation winding. This solution functions well for direct currents to be compensated in one range, where these direct currents are smaller by an order of magnitude than geomagnetically induced currents, i.e., approximately in the range below 10 A. For geomagnetically induced currents, the medium voltage level would have to be used, i.e., in the range of approximately 5 kV and high-powered thyristors used. Due to the high power loss of such thyristors this solution, however, is not economical.
A further solution for geomagnetically induced currents is represented by a “DC blocker” in which, in principle, a capacitor is connected to the neutral point of the transformer. This solution is problematic as a displacement voltage is produced by charging the capacitor. Moreover, the displacement voltage on the capacitor is limited so that it is generally not possible to block the entire direct current. A drawback with this solution is also when it results in a short circuit in the transmission network and therefore to zero currents.